J. Braz. Chem. Soc. vol.21 número7

Review

*e-mail: jtelser@roosevelt.edu

Overview of Ligand

versus

_{Metal Centered Redox Reactions in Tetraaza Macrocyclic}

Complexes of Nickel with a Focus on Electron Paramagnetic Resonance Studies

Joshua Telser*

Department of Biological, Chemical and Physical Sciences, Roosevelt University, 430 South Michigan Avenue, Chicago, 60605-1394 IL USA

Complexos de cobre(II) (3d9_{, S = 1/2) são estáveis e amplamente investigados por espectroscopia}

de ressonância paramagnética eletrônica (EPR). Já o isoeletrônico níquel(I) é muito menos comum e muito menos estudado. No entanto, níquel(I) tem interesse biológico, uma vez que o sítio ativo da metil coenzima M redutase (MCR) contém um ligante macrocíclico, F₄₃₀, que coordena o NiI

na sua forma ativa, MCR_red1. Assim, o comportamento redox e espectroscópico de complexos tetraazamacrocíclicos de níquel tem importância na química biomimética. O estudo desses complexos é complicado pela diiculdade na obtenção de NiI _{a partir dos precursores estáveis de Ni}II_{. A redução}

de complexos macrocíclicos de NiII _{pode gerar Ni}I _{em certos casos, mas em muitos outros leva à}

redução do macrociclo, gerando um ânion radical orgânico. Estudos anteriores da formação de complexos tetraazamacrocíclicos de NiI _{são aqui discutidos em termos da competição entre a redução}

centrada no metal e a centrada no ligante. Resultados de EPR são particularmente importantes para distinguir esses dois processos de redução, já que a formação de NiI _{produz espectros de EPR}

característicos, similares aos de CuII_{, enquanto a redução centrada no ligante gera espectros de EPR}

agudos, centrados em g = 2,00 e típicos de radicais orgânicos. Mesmo que uma redução centrada no metal ocorra, a geometria do complexo macrocíclico de NiI _{resultante é amplanente variável e,}

consequentemente, o espectro de EPR também será. Nesse caso, a comparação é entre os extremos dos espectros típicos de complexos tetragonais distorcidos (estado fundamentaldx2–y21, que inclui as

geometrias octaédrica tetragonalmente distorcida, piramidal de base quadrada e quadrado-planar) e dos complexos bipiramidais de base trigonal (estado fundamentaldz21). Trabalhos anteriores

realizados com CuII _{foram relacionados com a situação para Ni}I_{. Os diferentes tipos de espectros de}

EPR desses sistemas são discutidos especiicamente usando exemplos inéditos de vários complexos tatraazamacrocíclicos de níquel, incluindo F₄₃₀ e a própria MCR.

Copper(II) (3d9_{, S = 1/2) complexes are stable and widely investigated by electron paramagnetic}

resonance (EPR) spectroscopy. In contrast, isoelectronic nickel(I) is much less common and much less investigated. Nickel(I), however, is of biological interest as the active site of methyl coenzyme M reductase (MCR) contains a tetraaza macrocyclic ligand, F₄₃₀, which coordinates NiI _{in its active}

form, MCR_red1. As result, the redox behavior and spectroscopy of tetraaza macrocyclic complexes of nickel is of importance in biomimetic chemistry. Such efforts are complicated by the dificulty in generating NiI _{from their stable, Ni}II_{, precursors. Reduction of Ni}II _{macrocyclic complexes can}

afford NiI _{in certain cases, but in many other cases can lead instead to reduction of the macrocycle}

to generate an organic radical anion. Previous studies on the formation of tetraaza macrocyclic complexes of NiI _{are discussed in terms of the competition between metal-centered and}

ligand-centered reduction. EPR results are particularly important in making the distinction between these two reduction processes, as formation of NiI _{gives characteristic EPR spectra similar to those for Cu}II_,

while ligand-centered reduction gives narrow EPR spectra at g = 2.00, typical of organic radicals.

Even if metal-centered reduction occurs, the geometry of the resulting NiI _{macrocyclic complex}

is highly variable and, as a result, the EPR spectral appearance is highly variable. In this case, the comparison is between the extremes of spectra typical for tetragonally distorted complexes (dx2–y21

ground state, which includes tetragonally distorted octahedral, square pyramidal and square planar geometries) and those for trigonal bipyramidal complexes (dz21 ground state). Previous work on CuII

was related to the situation for NiI_{. The different types of EPR spectra for such systems are speciically}

discussed using previously unpublished examples of several tetraaza macrocyclic complexes of nickel, including F₄₃₀ and MCR itself.

the EPR spectra of a AgII _{porphyrin can be analyzed}

analogously to the corresponding CuII _complex.18 _Other

possibilities lie outside of Group 11. These could include Group 9 complexes in the zero oxidation state, e.g., Co0_;

however, such species are more realistically considered as organometallic radicals and are typically found in di- or polynuclear complexes, such as diamagnetic [Co₂(CO)₈].19

The most viable candidate is in Group 10, namely NiI_.

Relative to NiII_{, Ni}I _{is uncommon; however, pioneering}

work by Busch and co-workers20 _{has shown the accessibility}

of a variety of coordination complexes of NiI_{. At that}

time, NiI _{complexes were of interest only to coordination}

chemists; however, the discovery soon thereafter of the enzyme methyl CoM reductase (MCR) changed that situation dramatically.21-23 _{MCR catalyzes the inal step in}

methane generation by archaea, a process by which most of biogenic methane is created.22,24-26 _{MCR is found in several}

microorganisms, of which that from Methanothermobacter

marburgensis is the best characterized (the taxonomy of

these organisms is complicated and has been changed over the years; older papers on MCR refer to this organism as

Methanobacterium thermoautotrophicum strain Marburg).

MCR contains at the active site a prosthetic group comprising a unique macrocyclic ligand, known as F₄₃₀ (based on its maximum absorption wavelength), a diagram of which is shown below.27,28 _{In contrast to tetrapyrroles,}

F₄₃₀ is a monoanion and is much more saturated. Each pyrroline ring has signiicantly different substituents and is identiied by the letters A through D, so that the upper left ring in the diagram below is denoted A, the upper right (with lactam substituent) is B, the lower right is C, and the lower left (with cyclohexanone substituent) is D. F₄₃₀ is relatively thermally unstable and can epimerize to give the 12,13-diepimer of the propionic acid side chains on ring C; shown below with the ring designations.28,29

In the resting state, inactive enzyme, F₄₃₀ contains a NiII

ion, which is EPR silent at X-band, but has been studied by magnetic circular dichroism (MCD).30,31 _{However, the}

active form, MCR_red1, contains NiI_,32-35 _{as does a related}

form, MCR_red2.36 _{X-ray crystallography has been possible}

on the relatively stable, NiII _{forms of MCR,}37,38 _{but not}

on the reactive, NiI _{forms. The crystal structure of the}

pentamethylester of F₄₃₀, F₄₃₀M, has also been reported (as the 12,13-diepimer, since this is the thermally stable form; CSD code: KOBCEJ).39

1.3. Model complexes for MCR that are porphyrin-derived

The discovery of MCR led to a reawakening of interest in the coordination chemistry of NiI _{and speciically in}

model chemistry of MCR_red1. Synthesis of the full structure

1. Introduction

1.1. General background on electron paramagnetic resonance (EPR) spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy has been widely applied over the past six decades to the study of coordination complexes of the d block (transition metal) ions.1,2 _{Among the many possible d}n _electronic conigurations found, the d9 _{coniguration has been}

particularly well studied.1-6 _{This is the case for several}

reasons, chemical and physical. In the chemical context, the d9 _{coniguration is best represented by Cu}II_{, which}

forms a vast number of stable coordination complexes,7

many of which have biological relevance.3,8-10 _{In the}

physical context, the d9 _{(S = 1/2) coniguration is very}

amenable to study by EPR spectroscopy since there are no complications from intermolecular electron-electron interactions in mononuclear complexes. As long as the CuII _{sites are suficiently diluted, there are}

no intramolecular electron-electron interactions either, although these can be observed in undiluted solids.11

It should also be noted that EPR spectra of multi-CuII

centers can be intricate due to intramolecular exchange coupling.12 _{Equally important, the EPR spectra of d}9

systems are highly informative in terms of providing information on molecular geometry and chemical bonding. This utility was demonstrated many years ago for CuII _{coordination complexes by Maki and}

McGarvey,13,14 _{and a more qualitative analysis of Cu}II

EPR spectra has been very useful in bioinorganic chemistry.3 _{In contrast, mononuclear complexes with}

multiple electron/holes, however, such as those with the d8 _{electronic coniguration (Ni}II _{in many coordination}

environments, such as tetrahedral and octahedral), often exhibit complicated intramolecular electron-electron interactions that arise from spin-orbit and spin-spin coupling.1,15 _{These effects can lead to signiicant}

zero-ield splitting (zfs) and hence dificulty in obtaining EPR spectra at conventional microwave frequencies (i.e.,

X-band: ca. 9 GHz). Use of high frequencies (> 95 GHz)

combined with high magnetic fields (up to 25 T), however, can yield EPR spectra of such “EPR-silent” NiII

complexes, both four-coordinate16 _{and six-coordinate.}17

1.2. Background on nickel(I) and on methyl CoM reductase (MCR)

Other than CuII_{, what transition metal ions have the}

d9 _{electronic coniguration? Silver(II) is uncommon, but}

of the F₄₃₀ cofactor would be a daunting task; however, the salient features of the electronic structure of the NiI

ion can be reproduced by much simpler complexes. These include some of the relatively more saturated tetraaza macrocyclic complexes irst reported by Busch and co-workers,20 _{and of the relatively less saturated, porhyrinic}

complexes described by Fajer and Stolzenberg and their co-workers.40-50 _{Among these models, the most fruitful has}

been that of Ni with the ligand octaethyisobacteriochlorin (OEiBC), a diagram of which is shown below. The octaethyl substituents aid in solubility, but may have other electronic effects. The stereochemistry at the four saturated positions (reduced cis pyrrole (pyrroline) rings A (or C) and B (or

D), applying the F₄₃₀ nomenclature to the diagram below) that distinguish OEiBC from its standard porphyrin

analog octaethylporphyrin (OEP) is not speciied. The bacteriochlorin (OEBC) has reduced trans pyrrole rings

(i.e., rings A/B and C/D), but has been much less studied

in terms of Ni chemistry. In between the porphyrin and iBC/BC in terms of saturation is the chlorin, in which only one pyrrole has been reduced,51 _{also shown below with}

unspeciied stereochemistry.

There is also the “triply” reduced form, in which only one ring remains a pyrrole, known as octaethylpyrrocorphin (OEPC). The synthesis and crystal structure of [NiII_(OEPC)]

have been reported,52 _{but, to our knowledge, no investigations}

of its reduction chemistry have been reported.

Ni(OEiBC) is prepared in the NiII _{form (as are}

[Ni(OEP)], [Ni(OEBC)], [Ni(OEC)]), but can be reduced electrochemically,47 _{or by Na(Hg) amalgam in dry}

organic solvents to yield the NiI _{complex in solution,}

[Ni(OEiBC)]−_.53 _{Other Ni}I _{isobacteriochlorin (iBC)}

complexes, which contain the fused cyclohexanone ring of F₄₃₀, can be analogously prepared.44 _[Ni(OEiBC)]− _{has not}

only spectroscopic relevance to MCR_red1, but also exhibits reactivity that has some similarities to that of MCR.42,48

What is striking about the effectiveness of OEiBC as a model ligand for F₄₃₀ is how structurally different the two are. F₄₃₀ is a much more highly saturated and more lexible macrocycle than OEiBC,54 _{although porphyrinic}

macrocycles should not be thought of as the rigid disks by which they are so often depicted. Extensive studies by Ghosh and co-workers55,56 _{have probed the conformational}

lexibility and deformations on porphyrinic complexes. Even more puzzling is that among the various NiII

porphyrinic complexes, only iBCs are successfully converted into NiI_.44 _{The fully unsaturated, π-conjugated}

OEP complex of NiII _{yields a ligand-centered radical}

upon reduction,47 _{although for the chlorin analog, an EPR}

spectrum of [NiI_(OEC)]− _{can be transiently observed.}49

A complication with these complexes when undergoing chemical reductions is formation of phlorins, in which

meso positions are reduced. Stable, square planar (sq pl),

N

N N

N

Ni

H

O

CO2H

CO₂H CO₂H

HN O

H HO₂C

H2NOC

HO2C

F

N

CO₂H CO₂H 12

13

12,13-diepimer

C

A

B

D

N

N N

N

Ni

Ni(OEiBC)

H

H _H

H

N

N N

N

Ni

Ni(OEBC)

N

N N

N

Ni

Ni(OEC)

H

H H

H

diamagnetic NiII _{phlorins result eventually from reduction}

of both [Ni(OEP)] and [Ni(OEC)].49 _{Concerning the}

closer models to F₄₃₀, namely those with the fused cyclohexanone ring, in both their porphyrin and chlorin forms (shown below), reduction gives stable complexes well characterized in solution by EPR, optical, and X-ray spectroscopic techniques. For both of these complexes, the EPR spectra exhibit a very slight g anisotropy indicating

a small contribution from spin density on Ni 3d orbitals, however these species can by no means be considered as authentic NiI_.44

The ligand-centered reduction might be expected for the porphyrins (OEP and the F₄₃₀ model), since they are as different from F₄₃₀ as is possible in terms of π-conjugation and thus have the greatest availability of ligand-centered orbitals of suitable (low) energy to be electron acceptors. The fused cyclohexanone ring, while leading to a closer model for authentic F₄₃₀, appears to have no effect at preventing ligand-centered reduction. Renner et al.44 _also

prepared hexahydro- and octahydroporphyrins (structural diagrams shown below; note that there are two regioisomers of the hexahydroporphyrin (CSD code: KODHAM), depending on which one of the two meso alkenes is reduced;

both are reduced in the octahydroporphyrin shown on the right; the hydrogens added to the meso positions are not

shown). These tetraaza macrocycles are lessπ-conjugated than the iBCs and reproduce the structure of F₄₃₀ as closely as one could reasonably hope for, yet they yield even more purely ligand centered (π-anion) radicals upon reduction, as shown by EPR spectra that consist of a narrow signal at

g = 2.0029 (essentially the free electron value, g_e = 2.0023,

so that there are no d orbital contributions to the SOMO whatsoever).44

Although EPR spectroscopy is a convincing indicator of metal versus ligand-centered reduction, Renner et al.44 also

employed X-ray absorption spectroscopic methods (XAS, EXAFS) that independently show the reduction of NiII _to

NiI _{and the associated changes in Ni-N bond lengths. The}

larger NiI _{ion can be accommodated by a distortion in which}

two Ni-N bonds lengthen signiicantly, while the other two shorten slightly relative to the NiII _{parent complex. Thus}

the ability of the speciic macrocycle to adjust to the size changes in the nickel ion contributes what is in a sense a steric effect in determining the site of reduction.47

1.4. Model complexes for MCR that are saturated macrocycle-derived

If one then begins from the other direction, namely the totally saturated macrocycle 1,4,8,11-tetraazacyclo-tetradecane ([14]aneN₄, cyclam), and its variously methyl substituted analogs (speciically, Me₆[14]aneN₄, shown below), then NiI _{complexes result upon electrochemical}

reduction of the NiII _{parent complex.}20 _{This result is}

perhaps the only one that is readily expected since there are no ligand-based π MOs to act as electron acceptors. However, introduction of only minimal π-bonding into the macrocyclic ligand can lead to generation of ligand-centered, as opposed to metal-centered (i.e., NiI) reduction

products. The results are summarized in the scheme below, where “NiI_{” indicates metal-centered reduction (upper row}

of diagram) and “• −_{” indicates ligand-centered reduction}

(lower row). Lovecchio, Gore and Busch studied a number of other such complexes, however the scheme below depicts the salient macrocyclic ligand types.20 _Related

studies were subsequently performed by Gagné and co-workers57 _{on these and analogous complexes with borate}

linked bisdimine ligands (not shown). A number of these complexes were later studied by EXAFS by Furenlid et al.58

In this wide range of macrocyclic complexes, as long as the imino groups are fully π-isolated, then the reduction is metal centered; all that is necessary for ligand-centered reduction is to have a single conjugated α-diimine functionality.57

Complexes of NiII _{with acyclic, as opposed to macrocyclic,}

α-diimine ligands ([(R'N=C(R)C(R)=NR')MX₂];

M = NiII_{, Pd}II_{; X = halide, alkyl etc) are of great interest in}

their own right, due to their activity as alkene polymerization catalysts.59,60 _{It should also be noted that the related,}

β-diketiminate ligand (NacNac, (RC(=NR′)CH(=NR′)CR)−₎

has been widely used for a wide variety of d and p block metal ions, and many of these complexes have catalytic

N

N N N

CO2Me

Ni

O

N

N N N

H

CO2Me

Ni

H

O

F430 model porphyrin F430 model chlorin

N

N N N

H

CO2Me Ni

H H

H

O H

N

N N N

H

CO2Me Ni

H H

H

O H H

F430 model hexahydroporphyrin F430 model octahydroporphyrin ~

~ ~

activity as well.60,61 _{Bai et al.}62 _{provide an example of such}

a NiII _{complex, and also provide a comprehensive listing}

of references on β-diketiminato complexes. These workers have also isolated NiI_{β-diketiminato complexes,}63 _which

indicates that the β-diketiminate ligand is not reduced, despite its extensive π-conjugation. A point that to my knowledge has not been made before is that F₄₃₀ itself can be thought to contain a β-diketiminate group, as shown below in red, which is not the case for any of the [14]1,4,8,11-di- or tetraene complexes shown above.

1.5. Computational studies on tetraazamacrocyclic Ni complexes

There are no obvious “rules of thumb” for a simple coordination chemist to use as guidelines as to whether a given NiII _{complex with amino/imino ligands will be reduced}

to a NiI _{complex, or to an organic radical anion species. Only}

the extrema in terms of macrocyclic ligand π-conjugation can be easily deined in that no π-conjugation (e.g., fully

saturated [14]aneN₄) gives NiI_{, and maximum π-conjugation}

(e.g., fully unsaturated (aromatic) OEP) gives a

ligand-centered radical. This problem thus represents a potentially fruitful area for application of computational methods, and indeed such studies have already been performed on MCR/F₄₃₀ and related macrocyclic model systems.64-69

Of particular relevance is the very recent study by Ryeng, Gonzalez and Ghosh.69 _{These workers performed}

an extensive DFT study of a carefully selected series of

Ni hydroporphyrin complexes. These included chlorin, iBC, and BC ligands with no substituents, and each with tetramethyl and octaethyl substituents. Complexes with heteroatom substitution, i.e., oxa- and thiaporphyrins, which

have been studied experimentally,70 _{were also investigated}

computationally; however, these are not relevant to the present discussion which is limited to tetraaza complexes of Ni. As is characteristic of the Ghosh group, the results are very comprehensive. We point out here only that relative to the NiII _{parent complex, [Ni}I_(OEC)]− _{and [Ni}I_(OEBC)]−

are actually calculated to be at lower energy than their ligand-reduced forms (by 0.2-0.3 eV). Apparently, these forms are not suficiently stabilized in solution to persist indeinitely, suffering from other reaction pathways, such as phlorin formation. The calculation for [NiI_(OEiBC)]−_,

however, indicates that this form is much lower in energy (by 0.55 eV) than the ligand-reduced form, [NiII_(OEiBC•_)]−_,

which apparently leads to its stability in solution. This stability has allowed the full EPR/ENDOR spectroscopic characterization of [NiI_(OEiBC)]−_.50 _{This energetic result}

is more the consequence of relative instability of the

ligand radical anion than of relative stability of the NiI

form. Conformational lexibility in the OEiBC macrocycle relative to the more rigid OEC and OEBC (and presumably OEP) is the crucial factor in stabilizing the NiI _{form. This}

quantitative result from computations agrees with earlier, qualitative proposals.49,51

Wondimagegn and Ghosh68 _{had earlier studied F} 430 itself

and shown that this unique ligand has unique conformational characteristics that help support the NiI _{species observed}

by a variety of spectroscopic methods.40,41,50 _{Nevertheless,}

the situation with more reduced, and presumably more lexible, macrocyclic complexes, such as those studied by the groups of Busch,20 _{and Gagné,}57 _{has yet to be resolved.}

2. EPR Results for Tetraazamacrocyclic Ni

Complexes

2.1. Overview of case studies of individual Ni complexes

We describe here EPR studies on several macrocyclic complexes of nickel that span a variety of tetraaza macrocycle coordination. Also included are EPR spectra of the isolated MCR cofactor, F₄₃₀, in its reduced, NiI _form

(NiI_F

430), together with the holoenzyme form that contains

this species, MCR_red1. In the case of enzymes, introduction of magnetically active nuclei is often much more feasible than in model complexes. The anaerobic organisms that are the source of MCR can be grown on medium enriched in, e.g., 61_{Ni (I = 3/2, 1.13% natural abundance), whereas}

chemical synthesis using such isotopes is very expensive. N

N N N

Ni

[NiI_(Me 6[14]aneN4)]+

H H H H N N N N Ni H H N N N N Ni

[NiI_(Me

6[14]4,11-dieneN4)]+ [NiI(Me6[14]1,4,8,11-tetraeneN4)]+

N

N N N

Ni

[NiII_(Me

2[14]1,3-dieneN4) ]+

H H N N N N Ni

[NiII_(Me

6[14]1,3,7,11-tetraeneN4) ]+[NiII(Me4[14]1,3,8,10-tetraeneN4) ]+

Such isotopologs deinitively showed the role of nickel in MCR.22,32,35

Concerning the tetraaza macrocyclic model complexes for MCR, we irst present the EPR spectra of

tct-[NiI(OEiBC)]−_{, which represents the most unsaturated}

macrocycle to give a stable NiI _{species in solution; the}

ttt- and tct- isomers (see diagram below) gave identical

EPR results. No solid NiI _{OEiBC complex has been}

isolated. Surprisingly, to our knowledge, no crystal structure of [NiII_{(OEiBC)] has been reported (nor of}

[Ni(OEC)]), although structures of [NiII_{(OEBC)] (CSD}

code: DEGTAK52_{) and [Ni}II_{(OEP)] (several structures,}

of which the most recent has CSD code: NOEPOR0271_),

and [Ni(OEPC)] (CSD code: DEGSUD52_{) are known.}

However, the crystal structures of the PdII _{series [Pd(OEP)],}

[Pd(OEC)], and tct-[Pd(OEiBC)] have been reported by

Stolzenberg et al.72 The larger size of PdII allowed a better

probe of the effect of ring reduction than for the NiII _analogs.

Lastly, the crystal structures of the series [Ni(TMP)], [Ni(TMC)] and [Ni(TMiBC)] (where TMiBC = dianion of 5,10,15,20-tetramethylisobacteriochlorin, and analogously for TMP and TMC) have been reported;73,74 _{however, these}

tetrapyrroles have substituents unlike those of F₄₃₀ (i.e., at

the meso positions, rather than at the β positions (pyrroles/ pyrrolines)) and are considered here only in passing. The relative stability of [NiI_{(TMC,TMBC,TMiBC)]}−

versus [NiII_{(TMC,TMBC,TMiBC}•_)]− _{has been studied}

computationally by Ryeng et al.,69 _{who showed that}

ligand-centered reduction is energetically favored for the TMC and TMBC complexes, but is less favored (by ca.

0.55 eV; similar to the result for OEiBC) for the TMiBC complex. Despite this, we are not aware of any report of a NiI _{species upon reduction of Ni(TMiBC). Perhaps the}

recent work of Ryeng et al. 69 _{will inspire a reinvestigation}

of this process in the meso-substituted NiII tetrapyrrole

series.

Moving in the direction of greater saturation, we also describe studies on a nickel complex of a diene derivative of 1,4,8,1l-tetraazacyclotetradecane, Me₆ [14]4,11-dieneN₄ (formally 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene), in which there is no conjugation of the two imines, so that a NiI _{species is}

formed upon reduction.20,57,58 _{Two geometrical forms of this}

complex are found, rac and meso, as shown in the diagram

below, and each has been structurally characterized in the NiII _{state (CSD codes: KUGNEF (meso), MAZTNI02}

(rac)).75 The structure of only the meso form has been

determined for NiI _{(as [Ni(Me}

6[14]4,11-dieneN4)](ClO4),

CSD code: KINNOK).58 _{The speciic Ni}I _{solid state sample}

studied here was a mixture of these rac and meso forms;

this heterogeneity is maintained in solution.

The final isolable tetraaza macrocyclic complex to be described is that of the fully saturated ligand 1,4,8,1l-tetramethyl-1,4,8,1l-tetraazacyclotetradecane (tetramethylcyclam, tmc, [14]ane(NMe)₄),76 _{for which two}

stereoisomers are available as in the diagrams shown below. Crystal structures of a variety of [NiII_(tmc)]2+ _complexes,

several with axial ligands, but none with nitrile(s), have been reported; that most relevant to this study is

RRSS-[NiII_(tmc)](CF

3SO3)2 (CSD code: DONCAK),77

which is a rigorously sq pl complex. It must be noted that although isomerically pure [NiII_(tmc)]2+ _complexes

can be isolated, this isomeric integrity is not maintained upon reduction. Chemical reduction of either NiII _pure

isomer yields solutions containing both the RSRS- and RRSS-[NiI_(tmc)]+ _isomers.76,78 _{For solubility reasons, the}

RRSS isomer crystallized selectively, as RRSS-[NiI(tmc)]

(CF₃SO₃)•NaCF

3SO3 (CSD code: ZIMWUN),76 however

the solutions studied here contain both isomers, albeit in unknown proportion. At equilibrium in aqueous solution, the RRSS/RSRS ratio is roughly 3:1.78

Related studies by Meyerstein and co-workers79,80 _on

a variant of tmc with macrocycle methylation (oficially,

N

N N N

Ni

tct_-[NiI_(OEiBC)]−

ttt_-[NiI_(OEiBC)]−

N N

Ni

N

N N

N

Ni H

H

[NiI(Me6[14]4,11-dieneN4)]+

N

N N

N

Ni

H

H

1RS,4RS,7RS,8SR,11SR,14SR

)-1,4,5,5,7,8,11,12,12,14-decamethyl-1,4,8,11-tetraazacyclotetradecane; referred to herein as C-meso-[Me₆[14]ane(NMe)₄], or as Me₆tmc;

diagram shown below) also showed the stability of NiI_.

The structure of only the NiII _{form of this complex has}

been reported (as [NiII_(Me

6tmc)](ClO4)2; CSD code:

DUKPUU).81

Lastly, we describe the EPR spectra of the unstable species formed upon γ-irradiation of both [NiII_(OEiBC)]

and [NiII_{(OEP)] at 77 K. This cryoreduction technique, in}

which γ-irradiation ejects electrons from the appropriate solvent (various organic solvents, such as ethanol, or water/ glycerol) has been pioneered by Davydov and applied to a wide variety of metalloproteins, including diiron-oxo proteins,82 _{iron-sulfur proteins,}83 _{heme proteins,}84,85 _and

MCR itself.86 _{These new results show that it is possible}

to generate a NiI _{porphyrin, but that it can survive only}

at cryogenic temperatures. This work is analogous, but in striking contrast, to the studies using UV-irradiation in luid solution, followed by freezing in liquid nitrogen, which showed only the generation of an anion radical, [NiII_(OEP•−_)].49 _{In a related technique, radiolysis (pulsed}

or steady-state), using electrons generated by a linear accelerator, has also been used to generate NiI _{from tetraaza}

macrocyclic NiII _complexes.79,80

2.2. Sources of complexes described in case studies and experimental protocol

The complexes studied were obtained from a variety of sources. Samples of Methanothermobacter marburgensis

MCR_red1 were provided by Prof. Stephen W. Ragsdale (University of Michigan, Ann Arbor, MI, USA) and prepared by reduction with TiIII _{citrate as described previously.}87

Isolated native F₄₃₀ and its 12,13-diepimer were provided by Prof. Robert A. Scott (University of Georgia, Athens, GA, USA) and converted in his laboratory to their NiI _forms

by reduction with TiIII _{citrate as described previously.}50

RRSS-[NiI_(tmc)](CF

3SO3)•NaCF3SO3 was provided by Prof.

Charles G. Riordan (University of Delaware, Newark, DE, USA) and prepared in his laboratory following literature procedures.76 _[NiI_(Me

6[14]4,11-dieneN4)](ClO4) (mixture

of rac and meso forms) was provided by Dr. Etsuko

Fujita, Brookhaven National Laboratory, Upton, NY, USA and prepared in her laboratory following literature procedures.57,58 _{The Ni}I _{forms of these complexes were}

provided as solids and then dissolved under nitrogen atmosphere in dry n-butyronitrile/n-propionitrile (9:7 v/v),

which mixture forms a good glass for EPR spectroscopy. The complexes ttt- and tct-Ni(OEiBC) were prepared

and chromatographically separated by Dr. Mark W. Renner (Brookhaven National Laboratory) as described previously.50 _{The Ni}I _{forms of these complexes were}

generated in Dr. Renner’s laboratory by reduction using Na(Hg) amalgam in dry 2-methyltetrahydrofuran (2-Methf) solution,50 _{and shipped at low temperature for EPR}

measurements at Northwestern University. The complex [NiII_{(OEP)] was obtained from Porphyrin Products (now}

Frontier Scientiic, Logan, UT, USA).

EPR spectra at 9.0-9.7 GHz (X-band) of MCR_red1 samples were recorded by Dr. Yih-Chern Horng at the University of Nebraska, Lincoln, NE, USA on a Bruker ESP 300E spectrometer. EPR spectra at 34-36 GHz (K_a-band, often, but erroneously, referred to as Q-band) were recorded on a modified Varian spectrometer at Northwestern University, Evanston, IL, USA. Experimental conditions are given in the igure captions. The 35 GHz spectra were recorded under “passage” conditions,88 _{so that the signal appears as an absorption,}

rather than irst derivative lineshape. The igures generally present digital derivatives in addition to or instead of the original, passage spectrum so that the appearance is consistent with typical EPR spectra, such as those reported elsewhere for such NiI _species.

NiII_{(OEP) and Ni}II_{(OEiBC) samples in 2-Methf}

solution were γ-irradiated at 77 K. The irradiation was done by Dr. Roman Davydov, Northwestern University, using a Gammacell 200 60_{Co irradiator at the University}

of Chicago Pritzker School of Medicine, using procedures developed by him.82,84,89-91 _{The irradiated samples were}

maintained at 77 K (or lower) throughout the subsequent EPR spectroscopic measurements.

N N

N N

Ni

RSRS-[NiI(tmc)]+ RRSS-[NiI(tmc)]+

N N

N N

Ni

R

R R

R

S S

S S

N

N N

N

Ni

C-meso-[NiI(Me6[14]ane(NMe)4)]+,

All computer programs for EPR simulation (QPOWA, written originally by Belford and co-workers at the U. of Illinois, Urbana, IL, USA,92,93 _{and DDPOWH) and ligand}

field analysis (DSOXF, DDN package) are written in FORTRAN (g77) and are available from the author.

2.3. 35 GHz EPR Spectra of NiI_F

430 and [Ni

I_(OEiBC)]−

Among the various tetraaza macrocyclic complexes of NiI _{studied here, the simplest EPR spectrum is that}

for the most structurally elaborate macrocycle, namely F₄₃₀. Figure 1 presents 35 GHz EPR spectra of NiI_F

430

and tct-[NiI(OEiBC)]−_{. The EPR parameters for these}

and other NiI _{species are summarized in Table 1. Use of}

higher microwave frequencies, here 35 GHz, often reveals rhombicity that is not resolved at X-band (ca. 9 GHz). This

is indeed the case for [NiI_(OEiBC)]−_{, by comparison of}

Figure 1 to the published X-band spectrum (see Figure 12 in Renner et al.45), although careful EPR simulation

allowed these workers to extract the two components of g_⊥: g = [2.061, 2.083, 2.2025], which values are

essentially identical to those obtained from 35 GHz spectra:

g = [2.063, 2.080, 2.204].50 In contrast, the 35 GHz

spectrum of NiI_F

430 is as axial in appearance as its X-band

spectrum (see Figure 3 in Holliger et al.94).

It is interesting that, despite the potentially very lexible F₄₃₀ macrocycle54,95 _{with its vast variety of sidechains,}

including fused lactam (B) and cyclohexanone (C) rings, and the differences among the nitrogen donors (one is not conjugated with the other three), and the presence of two different Ni-N distances as determined by EXAFS,40 _the

EPR spectrum of NiI_F

430 is rigorously axial (with g|| = 2.244,

g_⊥ = 2.063) to within ± 0.002 in g value (ca. 1 mT at

35 GHz, g = 2.0). We suggest that this may be evidence that

the orientation of the in-plane components of the g matrix

(g_x, g_y) may be exactly bisecting the N-Ni-N bond angles,

so that an average value results. Single crystal studies of CuII _{complexes have shown that an orientation of g}

x, gy non-coincident with the Cu-N bond vector can occur.96,97

The 12,13-diepimer of NiI_F

430 was also investigated, but its

35 GHz EPR spectrum in our hands was indistinguishable from native NiI_F

430 (not shown), although a very slight

difference between the native and diepimeric forms has been reported.94 _{We have found that different preparations}

and/or slight differences in buffer/glassing agent of NiI_F 430

and of MCR_red1 give variations in g values (e.g., ± 0.005

in g_||) that is on the order of that reported for the diepimeric

versus native forms.

X-band EPR (and lower frequencies), however, can reveal hyperine splitting that is not resolved at higher ields/frequencies. The X-band EPR spectrum reported

for [NiI_(OEiBC)]− _{shows resolved hyperine coupling at} g_⊥ from the four, essentially equivalent, pyrrole/pyrroline

nitrogens (A(14N)_g⊥ = 0.98 mT, 28 MHz),47 which is not

seen at 35 GHz. The EPR feature at g_⊥ for [NiI_(OEiBC)]− _is

qualitatively very similar to that seen for CuII _{tetrapyrroles,}

such as [Cu(TPP)]18 _{or [Cu(OEP)] (A(}14_N)

g⊥ = 42 MHz).

98

Use of even lower microwave frequencies than X-band, such as S-band (ca. 1 GHz) or L-band (3 GHz), might

provide even better resolution of the 14_{N hyperfine}

splitting, as has been shown for CuII _{complexes by Hyde}

and Froncisz.4 _{In the case of Ni}I_F

430, the reported X-band

spectrum reveals only a hint of resolved hyperine coupling, although “massaging” of the data (Fourier-iltered second derivative presentation) did reveal hyperine coupling (A(14N)_iso = 1.0 mT, 29 MHz).94 The X-band spectrum

of NiI_F

430M (the organic-soluble, pentamethyl ester of

F₄₃₀) does show barely resolved 14_{N hyperine coupling}

with A(14_N)

g⊥ = 0.95 mT, 27 MHz.

33 _{The narrow range of} 14_{N hyperine coupling for these complexes indicates a}

commonality in bonding amongst them.

2.4. X-band and 35 GHz EPR Spectra of MCR_red1

An extensive discussion of MCR, with its many forms, both EPR-active and EPR-silent,24-26,99-101 _{is outside the}

scope of this study. We present here EPR spectra only of the form that is correlated with enzyme activity, MCR_red1,22,102 _{which resembles by EPR spectroscopy most}

closely NiI_F

430 and [NiI(OEiBC)]−.103 The 35 GHz spectrum

Figure 1. Experimental (dashed trace of pair (colored in online version)) and simulated (solid black trace of pair) 35 GHz EPR spectra of NiI_F

430

in aqueous solution and of [NiI_OEiBC]– _{in 2-Methf. The spectra were}

recorded at 2 K using the dispersion mode under passage conditions; a numerical irst derivative is shown. The abscissa is in g value to facilitate

comparison between spectra recorded at different frequencies (35.035 for NiI_F

430; [NiIOEiBC]- for 35.422 GHz). The simulation parameters for

NiI_F

430 are g|| = 2.244, g⊥ = 2.063, W|| = 140 MHz, W⊥ = 90 MHz (single crystal Gaussian linewidths, hwhm); for [NiI_OEiBC]−_{: g}

Table 1. Frozen solution EPR parameters for EPR-active MCR forms and tetraazamacrocyclic NiI _complexes

Complex, solvent g values

(g_max, g_mid, g_min)

A values (MHz) Reference

MCR_red1 a

aqueous buffer for all entries

MCR_red1c b

MCR_red1m c

MCR_red1a d

2.2745, 2.0820, 2.0680

2.2500, 2.0710, 2.0605

2.2467, 2.0671, 2.0598 2.2479, 2.0677, 2.0595

2.2515, 2.0730, 2.0635

2.224, 2.065, 2.057

14_N iso: 28 14_N

iso: 28 61_Ni

max: 195

14_N iso: 28 61_Ni

max: 195

61_Ni max: 200

99

99,101

25

99,101

This work MCR_red2 e

aqueous buffer for both entries

MCR_red2r e

2.2940, 2.2313, 2.1790

2.2880, 2.2348, 2.1790

2.2885, 2.2339, 2.1771 2.2886, 2.2339, 2.1797

14_N iso: 24.6

61_Ni max: 67

99

101

25

MCR_ox1 f

aqueous buffer 2.2310, 2.1667, 2.1532

2.2312, 2.1678, 2.1527

14_N iso: 27

14_N iso: 27.1 61_Ni

max: 132

99,101

24

NiI_F 430

aqueous buffer g_|| = 2.224, g_⊥ = 2.061

g_|| = 2.244, g_⊥ = 2.063

14_N iso: 29

14_N iso: 30

94

50 NiI_F

430M g

thf 2.250, 2.074, 2.065 14_N

iso: 27 33

Complex, solvent g values

(g_max, g_mid, g_min)

A values (MHz) Reference

[NiI_(OEiBC)]− 2-Methf thf

2-Methf

γ-irradiation product h

2-Methf

γ-irradiation product, warmed

g_|| = 2.201, g_⊥ = 2.073

2.2025, 2.083, 2.061

2.204, 2.080, 2.063

g_⊥ = 2.075, g_|| = (2.11, 2.18, 2.30)

major: 2.195, 2.105, 2.095; minor: g_⊥≈ 2.05(5), g_|| = 2.30

14_N iso: 28 14_N

iso: 28

14_N iso: 30

47 45,50

This work

This work [NiI_(OEC)]− _(unstable) i

thf 2.19, 2.10, 1.98 49

[NiI_(OEP)]− _(unstable) j γ-irradiation product

2-Methf 2.179, 2.093, 2.090 This work

[NiI_(Me

6[14]4,11-dieneN4)]

+ k

MeCN

propylene carbonate

n-PrCN/EtCN 7:3

[NiI_(Me

6[14]4,11-dieneN4)(CO)]+

g_|| = 2.226, g_⊥ = 2.055 g_|| = 2.220, g_⊥ = 2.063

57%: 2.256, 2.060, 2.042; 43%: 2.229, 2.182, 2.022

2.201, 2.123, 2.018

20 57

This work

57 [NiI_(tmc)]+ l

n-PrCN/EtCN 7:3 69%: 2.352, 2.220, 2.032;

10%: 2.285, 2.205, 2.088; 21% 2.343, 2.230, 2.0061

Complex, solvent g values (g_max, g_mid, g_min)

A values (MHz) Reference

[NiI_(Me

6[14]aneN4)]+

MeCN

propylene carbonate

[NiI_(Me

6[14]aneN4)(CO)]+

propylene carbonate

g_|| = 2.266, g_⊥ = 2.055

g_|| = 2.253, g_⊥ = 2.054

2.198, 2.123, 2.012

20 57

57

Complex, solvent g values

(g_max, g_mid, g_min)

A values (MHz) Reference

[NiI_(tmc)(O 2)]

+

dmf/toluene 1:2 2.29, 2.21, 2.09 108

a _{Red1 EPR signal present in the MCR}

red2 sample showing a mixture of red2 and red1 signals;99 in the red2 form, the NiI is coordinated by the thiol(ate)

sulfur of coenzyme M (HSCoM = HSCH₂CH₂SO₃−_).36b _{Red1 EPR signal in the presence of coenzyme M; this signal is referred to as MCR}

red1c. c Red1

EPR signal in the presence of methyl-coenzyme M (CH₃SCH₂CH₂SO₃−_{); this signal is referred to as MCR}

red1m. d Red1 EPR signal in the absence of other

substrates, coenzymes, or other forms of MCR. This signal is denoted MCR_red1a, but simulation parameters for this speciic form could not be found in the relevant references of Thauer and co-workers.99-101e _{Red2 signal as originally reported; this is now referred to as MCR}

red2r (r = rhombic); there is also an

axial red2 signal denoted MCR_red2a, with EPR signals very similar to MCR_red1a.25 _{Slightly different EPR parameters result depending on the two methods}

of generation of MCR_red2r;25 _{both parameter sets are given here.} f _{Ox1 signal as originally reported. Harmer et al. determined the full A(}14_{N) tensor for all}

four nitrogens of the macrocycle;24 _{the average, isotropic value of them all is given here for comparison with less reined data. In MCR}

ox1, there is formally

a NiIII _{ion with thiolate ligation from CoM [Ni}III_-(RS−_{)]; this can also be considered as a spin-coupled Ni}II _{-thiyl system, [Ni}II_-(RS•_)].64g _{Pentamethyl ester}

of F₄₃₀.33h _{(2,3,7,8-tetrahydro-2,3,7,8,12,13,17,18-octaethylporphyrin) Ni}II _{(octaethylisobacteriochlorin) cryoreduced by γ-irradiation; the resulting Ni}I

species is heterogeneous, with several features that can be assigned to g_||. The second entry is for the sample after brief (ca. 5 s) warming to 300 K under nitrogen. i _{(2,3-dihydro-2,3,7,8,12,13,17,18-octaethylporphyrin) Ni}II _{(octaethylchlorin) chemically reduced by sodium tetracenide; the resulting Ni}I _species

is unstable towards formation of a chlorin-phlorin anion.49j _{(2,3,7,8,12,13,17,18-octaethylporphyrin) Ni}II _{cryoreduced by γ-irradiation; the Ni}I _signal

disappears upon brief warming to 300 K under nitrogen. k _{Mixture of meso and rac forms of}

(5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) NiI _perchlorate.58 _{The relative amount of the two isomers is unknown.} l _{Mixture of RRSS and RSRS isomeric forms of}

(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) NiI _{(tmc) triluoromethylsulfonate. The relative amount of the two isomers is unknown, but is ca. 3:1 at equilibrium}

in aqueous solution.78m _{(C-meso-1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11-tetraazacyclotetradecane) Ni}II _(Me

6[14]ane(NMe)4, Me6tmc) reduced by

steady-state radiolysis.79

Table 1. Frozen solution EPR parameters for EPR-active MCR forms and tetraazamacrocyclic NiI _{complexes (cont.)}

of MCR_red1 is shown in Figure 2, for both natural isotopic abundance and 61_{Ni-enriched samples. The EPR spectrum}

of the natural-abundance sample is almost the same as that for NiI_F

430, with g = [2.224, 2.065, 2.057], indicating

that the electronic structure about the NiI _{ion, including}

the nearly axial symmetry, is the same in the protein as in the isolated cofactor. In the enriched enzyme sample, hyperine splitting due to 61_{Ni (I = 3/2) is apparent at g}

||,

but is essentially unobservable at g_⊥. The use of 61_{Ni thus}

yields an EPR spectrum for MCR_red1 that resembles that for typical tetragonally distorted six-coordinate (square pyramidal, sq pyr) CuII ₍63,65_{Cu, I = 3/2, 100% abundance)}

with dxz,yz4dxy2dz22d_x2–y21coniguration. The natural abundance

35 GHz spectrum reveals very slight rhombic symmetry at g_⊥, but the resolution of the hyperine splitting at g_|| in

the enriched sample is less than ideal. However, X-band EPR provides good resolution of A(61_Ni)

|| = 200(10) MHz,

equivalent to the value reported elsewhere.99,101 _This

result is analogous to the improved resolution at X-band compared to 35 GHz of 14_{N hyperine described above}

for NiI_F

430 and [NiI(OEiBC)]−. The chief difference

between the 61_{Ni-enriched spectrum for MCR} red1 and

that for typical CuII _{tetrapyrroles is the larger magnitude}

hyperine coupling in the latter (e.g., A(63Cu)_|| = 630 MHz

for [Cu(OEP)]98_{). This three-fold larger A value in the Cu}II

complex is largely the consequence of the three-fold larger

g_N for Cu (63_{Cu, g}

N = 1.484; 65Cu, gN = 1.588) versus61Ni

(g_N = −0.500),5 _{so that the bonding in the two complexes}

is actually quite similar (A/g_N = 400 for 61_{Ni; 424 for} 63_Cu).

Indeed, the M-N bond distances are also quite similar. The CuII_{-N distances in [Cu(OEP)] are 199.6(3) pm}

(Cu-N(1)) and 199.9(3) pm (Cu-N(2)),104 _{while the}

EXAFS-determined NiI_{–N distances in [Ni}I_(OEiBC)]− _are

two at 191 pm and two at 207 pm,45 _{and for Ni}I_F 430M,

two at 188 pm and two at 203 pm.40 _{It is unfortunate that}

cost precludes 61_{Ni hyperine coupling data from being}

more available for NiI _{complexes in general, but the EPR}

results for MCR_red1 clearly show that this species, and by extension, isolated NiI_F

430, whether in aqueous or organic

solvent, and [NiI_(OEiBC)]−_{,are all typical tetragonally}

distorted (whether square planar (sq pl), square pyramidal, or even six-coordinate, is not signiicant) dxy,xz,yz,z28dx2–y21

complexes, such as commonly found for CuII _with

2.5. 35 GHz EPR spectra of [NiI_(Me

6[14]4,11-dieneN4)]

+

The complex [NiI_(Me

6[14]4,11-dieneN4)](ClO4)

represents a step away from π-conjugation relative to the species discussed above. It is one of the few NiI

complexes to be crystallographically characterized (in the racemic form) and was thus used for bond distance calibration in EXAFS studies of NiI _{species for which no}

crystal structures were available (e.g., NiIF₄₃₀).58 It is also

representative of the many tetraaza macrocyclic complexes described by Busch and co-workers.20 _{The X-band spectrum}

of this electrochemically generated complex in propylene carbonate frozen solution was reported by Gagné and Ingle57 _{and gave g}

|| = 2.220, g⊥ = 2.063. As can be seen

from Table 1, these values are totally unremarkable, and indeed, are almost the same as those for the NiI _species

described in the preceding sections. Here, however, a mixed nitrile solvent system was used (n-butyronitrile/

propionitrile, 7:3 v/v), which provides a good glass for EPR. This nitrile solvent system is effective at dissolving the ionic complex and nitriles would be expected to be relatively weak donors, compared to, e.g., CO, the binding

of which had been extensively studied in NiI _complexes.57

Nevertheless, an EPR spectrum quite different from that of the NiI _{species discussed hitherto results, as shown in}

Figure 4. The signal is clearly heterogeneous, and can be adequately described as the superposition in roughly equal amounts of two signals, one described by g = [2.256,

2.060, 2.042], and one with g = [2.229, 2.182, 2.022]. The

former, nearly axial g matrix, while different from those

previously reported,20,57 _{is nevertheless similar to that for the}

other tetragonal NiI _{complexes described herein (Table 1).}

The better ield dispersion of 35 GHz EPR might allow resolution of rhombic splitting that was not observable in the earlier X-band studies,20,57 _{and the difference in solvent}

might be responsible for the other differences – note the variation in g values among the various forms of MCR_red1

and of NiI_F

430 – all in aqueous solvent (Table 1). Note

also that the crystal structure of meso-[NiI(Me₆

[14]4,11-dieneN₄)](ClO₄) shows a planar NiN₄ unit with two sets of Ni-N bond distances,58 _{which would be expected to give a}

slightly rhombic, tetragonal type (g_||ca. 2.2 > g_⊥ca. 2.05)

of EPR signal. The EPR signal with the axial g is thus

assigned to a typical, tetragonal NiI _{tetraaza macrocycle:}

dxz,yz4dxy2dz22dx2–y21 ; sq pl in the absence of any axial ligand

(from solvent) coordination; ive-coordinate with one axial ligand; six-coordinate with two, all analogous to CuII

complexes of the same geometry.

What about the rhombic signal? Such a signal is similar to that seen for MCR_red2: g = [2.2940, 2.2385, 2.1790].99

In this MCR form, there is an axial sulfur donor (from Figure 2. Experimental (dashed trace of pair (colored in online version))

and simulated (solid black trace of pair) 35 GHz EPR spectra of MCR_red1 in natural isotopic abundance and 61_{Ni-enriched. The spectra were recorded}

at 2 K using the dispersion mode under passage conditions; a numerical irst derivative is shown. The abscissa is in g value to facilitate comparison

between spectra recorded at different frequencies (35.035 for natural abundance; 34.945 GHz for 61_{Ni-enriched). The simulation parameters}

are g = [2.224, 2.065, 2.057], W = 90 MHz (single crystal Gaussian linewidths, hwhm); the enriched sample includes: A(61_Ni)g

max = 200 MHz.

Figure 3. Experimental (dashed trace of pair (colored in online version)) and simulated (solid black trace of pair) X-band (9.47 GHz) EPR spectra recorded at 77 K of MCR_red1 in natural isotopic abundance and

coenzyme M) to the NiI _ion.25,36 _{Perhaps more relevant,}

exposure to CO leads to formation of [NiI_(Me

6

[14]4,11-dieneN₄)(CO)]+ _{with g = [2.201, 2.123, 2.018].}57 _One

could propose therefore, that the highly rhombic signal observed for [NiI_(Me

6[14]4,11-dieneN4)]+ results from

axial coordination by a nitrile involving π-donation from NiI _{to the axial ligand, as with Ni}I_{−CO bonding. That a}

nitrile could have this effect would be a statement as to the powerful π-donor abilities of NiI_{, which is related to}

its nucleophilic role in MCR action. However, previous EPR studies on NiI _{complexes showed no such behavior}

in acetonitrile solvent.20 _{It is apparently the case here}

that the lexible macrocyclic ligand, whether the cause or effect of nitrile binding, adopts a conformation that is highly distorted from square planar tetraaza (overall square pyramidal due to one axial nitrile, CO, thiol(ate) etc), becoming trigonal bipyramidal (tbp) in the extreme case. For ideal tbp geometry, which for d9 _{has the electronic}

conigurationdxz,yz4dxy,x2–y24d_z21, the g values are: g

||≅ 2.00 <

g_⊥≅ 2.25(5).105,106 _{Such an axial signal is not seen here,}

but the lower symmetry present in these NiI _{complexes is}

unlikely ever to yield ideal tbp geometry.

What about intermediate geometries? This situation is much more complicated, but has been beautifully worked out using ligand-ield theory by Bencini and co-workers.105,106 _{This theoretical work was in conjunction}

with their EPR studies on bis(N-methylsalicylaldiminato)

complexes of CuII_{, which quinquidentate ligand strongly}

favors tbp coordination geometry. Bencini et al.105,106

provided equations for the g tensor components for the

entire transition from square pyramidal to tbp in C_2v

symmetry. They explain (especially see Figure 4 in Bencini

et al.106) that this change causes g_max (g_z) slightly to decrease

from roughly 2.30 to 2.20; g_min (g_x) likewise decreases also

only slightly, from 2.07 to 2.00; however g_mid (g_y) varies

signiicantly during this transition, from roughly 2.07 to 2.22.

Such a geometry that approaches tbp could thus be proposed for the second species in [NiI_(Me

6

[14]4,11-dieneN₄)]+_{, that with g = [2.229, 2.182, 2.022]; this g}

could correspond approximately to αca. 115o, where 90o

(sq pyr) ≤α≤ 120o _{(tbp). Equations for A(}63,65_{Cu) were also}

given,106 _{which should be applicable to} 61_{Ni. Unfortunately,}

there are no A(61_{Ni) data to which to apply the Bencini}

model except for MCR_red1, which its their model of a typical tetragonal/square pyramidal system. One would expect that

61_{Ni-enriched [Ni}I_(Me

6[14]4,11-dieneN4)]+ would show

large (ca. 200 MHz) 61_{Ni hyperine coupling at g}

max (gz) for the axial (sq pyr) signal and smaller for the rhombic (tbp) signal. We further speculate that the rac form corresponds

to the rhombic EPR signal, as this form binds CO,57,58 _while

the meso form corresponds to the axial EPR signal, similar

to the structurally characterized form.

2.6. 35 GHz EPR spectra of [NiI_(tmc)]+

The final, stable NiI _{species to be described here}

is that with the fully saturated tetraaza macrocyclic ligand, tmc. The crystal structure of RRSS-[NiI_(tmc)]

(CF₃SO₃)•NaCF

3SO3 shows that the geometry around

the NiI _{ion is exactly planar with two sets of Ni−N bond}

distances (209.5 and 212.0 pm),76 _{analogous to the results}

for [NiI_(OEiBC)]−_{. The EPR spectrum of [Ni}I_(tmc)]+ _has

not, to our knowledge, been previously reported. However, the spectra for other fully saturated tetraaza macrocyclic complexes of electrochemically generated NiI _have

been reported, such as with Me₂[14]aneN₄ (g_|| = 2.261, g_⊥ = 2.060) and Me₆[14]aneN₄ (g_|| = 2.266, g_⊥ = 2.055 in

acetonitrile solution; g_|| = 2.253, g_⊥ = 2.054 in propylene

carbonate solution).20,57 _{These EPR parameters are again}

very similar to many other such tetragonal/sq pyr/sq pl complexes (see Table 1).

In contrast to these clear-cut, earlier results, the 35 GHz EPR spectrum of [NiI_(tmc)]+ _{in the nitrile solvent system is}

heterogeneous. As shown in Figure 5, the spectrum can be deconvoluted into at least two components, or better with three. Two components are expected since, although the solution was prepared from solid RRSS isomer, in solution there is

Figure 4. Experimental (dashed trace (colored in online version)) and simulated (solid black trace) 35 GHz EPR spectrum of [NiI_(Me

6

[14]4,11-dieneN₄)](ClO₄) in n-butyronitrile/propionitrile (7:3 v/v) frozen solution. The spectrum was recorded at 2 K (and 35.116 GHz) using the dispersion mode under passage conditions; both the experimental absorption lineshape (lower dashed (colored) trace) and a digital irst derivative (upper dashed (colored) trace) are shown with their simulations. The simulation is the sum of two parameter sets: 57% integrated intensity weighting using g = [2.256, 2.060, 2.042], W = [100, 60, 60] MHz (single crystal Gaussian linewidths,

inter conversion so that the RSRS isomer is also present.76,78

The relative amount of the two isomers in nitrile solutions is unknown, but at equilibrium is ca. 3:1 in aqueous solution,78

so that the deconvolution into 69% major component and 31% two minor components is not that far off from the aqueous solution result. However, these three EPR components are all highly rhombic and none resembles typical tetraaza macrocyclic complexes of NiI _{(i.e., Cu}II_{-like parameters:}

g_||≅ 2.25(5), g_⊥≅ 2.05(5); see Table 1), as would be expected from the crystal structure. A possible explanation is that axial coordination of the nitrile ligand leads to formation of species that are electronically very similar to the CO adducts of the NiI

macrocycles reported by Gagné and Ingle.57 _{This possibility}

was raised above to explain the rhombic component in the EPR spectrum of [NiI_(Me

6[14]aneN4)]+ (Figure 4). It is surprising,

however, that butyronitrile/propionitrile would behave as the strong π-acceptor CO does. Furthermore, although Gagné and Ingle used the polar, but totally non-coordinating solvent propylene carbonate,57 _{acetonitrile was employed earlier by}

Lovecchio, Gore, and Busch,20 _{and their spectra differ only}

trivially from the corresponding spectra reported by Gagné and Ingle.57 _{The rhombic signals seen for [Ni}I_(tmc)]+ _also

strongly resemble those for MCR_red2,36,99 _{however, these result}

from an axial thiolate ligand (from coenzyme M) to NiI_{, and}

no such species is available in the present case. Meyerstein and co-workers79,80 _{used radiolysis, as well as electrochemistry,}

to generate tetraaza macrocyclic complexes of NiI _{from Ni}II

in aqueous solution. They reported EPR spectra at 77 K of radiolytically generated [NiI_(Me

6tmc)]+ that were typical for

a tetragonal complex (g_|| = 2.333, g_⊥ = 2.069; see Table 1);

however, in the presence of formate ion, highly rhombic spectra were observed: g = [2.261, 2.136, 2.073].79 No

explanation for this was given.

Our speculation for the EPR behavior of [NiI_(tmc)]+ _in

nitrile frozen solution, and possibly the results of Jubran et al.,79 is the same as that given above for [NiI(Me₆

[14]4,11-dieneN₄)]+_{, namely that there is distortion away from sq}

pl or sq pyr (with axial nitrile) geometry towards either distorted tetrahedral or tbp geometry (with equatorial nitrile), which leads to mixing in of dz21 character into the

ground state. The difference among the three forms seen by EPR is relatively slight; we can only speculate the one form corresponds to one isomer in a given geometry, whether distorted tetrahedral or tbp (due to nitrile coordination), and the other two to the other isomer in each of these geometries (or tbp with both axial and equatorial nitrile coordination).

2.7. Discussion of “nickel(I)-dioxygen” species

Solution samples of the NiI _{complexes that were}

provided as solids, meso, rac-[NiI_(Me

6[14]aneN4)]+ and

RRSS-[NiI(tmc)]+_{, were prepared under inert atmosphere.}

However, the possibility that some amount of dioxygen adducts were formed cannot be totally excluded. We therefore summarize here very interesting and recent studies by Riordan and co-workers107 _{on dioxygen binding}

to NiI _{complexes, including RRSS-[Ni}I_(tmc)]+ _{in a variety of}

solvents (e.g., MeCN, thf, dmf and MeOH).108 A complex

they formulated as [Ni(tmc)(O₂)]+ _{exhibited a rhombic EPR}

signal (X-band, 14 K) with g = [2.29, 2.21, 2.09],108 which

is remarkably similar to those for CO adducts of NiI _tetraaza

macrocycles.57 _{However, a wide variety of other physical}

techniques were used to characterize this dioxygen complex in solution, including UV-Vis, XAS, and Resonance Raman spectroscopy. Such a species can have multiple descriptions: [NiI_-O

20]+ (dioxygen), [NiII-O2−]+ (superoxo),

or [NiIII_-O

22−]+ (peroxo), which we will evaluate here.

The NiIII_{-peroxo formulation would be expected to}

give EPR spectra typical of such low-spin 3d7 _complexes

(for tetraaza macrocyclic complexes of NiIII_{: g}

||≅ 2.02(2),

g_⊥ ≅ 2.20(2)20,109_{), which is similar to that of [Ni(tmc)}

(O₂)]+ _(g

|| = 2.09, g⊥ = 2.25(4)). The EPR spectrum of

[Ni(tmc)(O₂)]+ _{is optimal at low temperature (6 K) and}

decreases with higher temperature (see Figure S9 in Kieber-Emmons et al.108_{). In contrast, EPR spectra for}

authentic NiIII _{complexes are readily observed even at}

room temperature.109 _{This suggests to us that the Ni}III

-peroxo description (which was disfavored based on other Figure 5. Experimental (dashed trace (colored in online version)) and

simulated (solid black trace) 35 GHz EPR spectrum of RRSS-[NiI_(tmc)]

(CF₃SO₃)•NaCF

3SO3 in n-butyronitrile/propionitrile (7:3 v/v) frozen

solution. The spectrum was recorded at 2 K (and 35.198 GHz) using the dispersion mode under passage conditions; both the experimental absorption lineshape (lower dashed (colored) trace) and a digital irst derivative (upper dashed (colored) trace) are shown with their simulations. In the left panel, the simulation is the sum of two parameter sets: 87% integrated intensity weighting using g = [2.352, 2.220, 2.032], 13% weighting using g = [2.285, 2.205, 2.088], for both W = [100, 80, 60] MHz (single crystal